1. Field of the Invention
This invention relates to a DC-DC converter, specifically to a DC-DC converter having a function to step down an input voltage.
2. Description of the Related Art
DC-DC converters are circuits that change a certain DC input voltage into another DC voltage, and are used in a power supply circuit of an LSI and the like. Among the DC-DC converters, a step-down DC-DC converter that has a function to step down an input voltage is well known.
This type of DC-DC converter is explained hereinafter referring to FIGS. 4A and 4B. C1-Cn are n capacitors switchable between a series connection and a parallel connection. Cout is an output capacitor (smoothing capacitor). The DC-DC converter is structured so that the n capacitors C1-Cn can be alternated by a switching circuit (not shown) between a status of series connection as shown in FIG. 4A and a status of parallel connection as shown in FIG. 4B. The status shown in FIG. 4A is called phase 1 and the status shown in FIG. 4B is called phase 2 hereafter. All the capacitors C1-Cn have the same capacitance.
The capacitors C1-Cn are connected in series between an input voltage Vin (DC voltage) and a ground voltage Vss (0V) in the phase 1. In this phase, the capacitors C1-Cn are charged with electric charges corresponding to a voltage (1/n)·Vin that is Vin multiplied by 1/n. In the phase 2, on the other hand, the capacitors C1-Cn are connected in parallel between the ground voltage Vss (0V) and an output terminal to which electric charges stored in the capacitors C1-Cn are discharged. An output voltage Vout obtained at the output terminal is represented by the following equation, Vout=(1/n)·Vin, which means that the input voltage multiplied by 1/n is obtained at the output terminal. Therefore, a stepped down voltage that is the input voltage Vin divided by n is obtained as the output voltage Vout by alternating between the phase 1 and the phase 2. Further description on the technologies mentioned above is found in Japanese Patent Application Publication No. H09-163719, for example.
However, the DC-DC converter described above requires n capacitors in order to multiply the input voltage Vin by 1/n, causing a problem of increased cost.